Flowering rosette of the extremely rare Mauna Kea silversword (Argyroxiphium sandwicense subsp. sandwicense growing with a shrubby or tree-like member of the Hawaiian silversword alliance (Dubautia arborea) at about 2,950 m elevation. In spite of great morphological differences (including unbranched monocarpic rosette shrubs, highly branched polycarpic shrubs, trees, and vines), virtually all 28 species in the three genera constituting the silversword alliance retain the ability to hybridize, and many striking hybrid combinations are produced in nature.

Autoradiograph of a small area of longisection of henbane anther following in situ hybridization with 35s-labeled rice histone antisense probe. Autoradiographic silver grains are localized in epidermis and endodermis (at bottom).

Transverse section, one locule of vigna vexillata (Fabaceae) anther. The three developing pollen grains are at the microspore stage; each has ektexine ridges overlying a thick inner exine wall (endexine). An additional plug of wall material (Zwischenk

Left panel: a germinated Chara oospore (zygote) with a rhizoid and protonemal cell. Right panel: Chara rhizoids that were generated from thallus cuttings. In both panels, the rhizoids are oriented relative to the gravity vector which is toward the bottom of the figures.

Autofluorescent cell walls of the connective bases of three anthers of a thistle, Cirsium horridulum. The stamen filaments extending down from each connective base are not visible because they lack the autofluorescent component in their cell walls.

At left, female (top) and hermaphrodite (bottom) flower of bladder campion (Silene vulgaris); their nectaries differ in production of sugar content. At right, a European skipper (Thymelicus lineola) removes nectar from a female bladder campion flower. Differences in nectar production between the genders of plants may affect pollinator activity and, ultimately, indbreeding.

The Bitterroot Valley of western Montana, showing a typical habitat for the rare species, Arabis fecunda: dry and rocky with erodible, sparsely vegetated slopes. This species has a mixed mating system and exhibits inbreeding depression, attributes of reproductive biology that may complicate efforts to preserve its populations.

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